The present invention relates to a diversity method in a radio system in which the duration of fadeouts is typically in the magnitude of seconds, a radio receiver intended for use in such a radio system, and a radio system.
Fadeouts are a common problem in radio transducer systems. Many different methods have been developed to alleviate this problem, with various diversity methods being the most important. In digital radio systems, channel coding, interleaving and frequency hopping are also often used to lessen the effects of fadeouts. The last-mentioned methods are commonly used in digital mobile communication systems, such as the European GSM system.
The present invention also relates to switching position diversity reception in which several different antennas, usually two antennas, are located in different places sufficiently and suitably distant from each other with regard to transmission wave length, and one of the optionally selectable antennas is connected to a receiver. Several different strategies are available for selecting one of the antennas. The simplest and commonly used method is to switch to a second antenna and begin receiving its signal whenever a signal received from a first antenna becomes weak. The antennas are located so that the signal quality of the second antenna predictably will be better. In a searching switching method, connections are rapidly made between several antennas until the signal quality of one of the antennas exceeds a specified quality limit, whereupon reception of said signal begins. In a selective diversity method, which is an alternative to the switching diversity method, the best signal is always selected. A disadvantage of this method is that several receiver circuits are required to simultaneously examine the quality of received signals. Therefore, this method easily becomes expensive.
FIG. 1 presents a solution based on the switching diversity method. This method is suitable for use in a radio receiver in which fadeouts occur in rapid succession. Such a situation often exists, for example, in a moving mobile communication terminal. In the solution of FIG. 1, a signal is available to a radio receiver 4 from two different antennas 1 and 2. A switch 3, which is used to select a signal from one antenna or the other, is controlled by means of a diversity method. The signal quality is measured in block 18, and the short-term average quality of the signal is calculated in block 5. Additionally, the long-term average quality is calculated in block 6 and multiplied by a selected weighting coefficient in block 7. The short-term average represents the momentary quality of the signal, and the weighted long-term average represents the signal quality limit that the momentary signal quality value is compared to in block 8. If the short-term average is lower than the weighted long-term average, a control generated by the comparison block 8 causes a switch 3 to select another antenna. In time-divided digital systems, in which a signal is received as short bursts at certain intervals, it is advantageous to use the signal quality measured from each burst as the short-term average. The long-term average signal quality is calculated over a long enough period to ensure that it truly represents the average quality of the radio channel. The parameter that depicts signal quality can be, for example, signal strength, signal-to-noise ratio, bit error ratio or mean square error.
In fixed wireless systems, which are often referred to as WLL systems (WLL, Wireless Local Loop) and in which a fixed telephone network is realized by means of radio connections and cellular nets, a terminal is located in a specific place, where it is completely stationary or its movement is very limited, in like manner as a terminal of a normal public telephone network. In such systems, fadeouts seen by a receiver are primarily caused by moving objects that reflect radio waves, and fadeouts develop slowly and last long. The duration of a fadeout may typically be in the magnitude of seconds, and fadeouts may not disappear until the reflecting object has moved sufficiently. In such a case, channel coding may quit functioning in a digital transmission. Interleaving and channel coding are not able to guarantee adequate performance on a channel that is susceptible to this type of fadeout. In this conjunction it must be noted that subscribers equate WLL systems with a fixed wire telephone network, placing the same quality requirements on it, and the fadeout phenomenon is unacceptable to them. Therefore, it is quite probable that diversity reception will be required in the terminals of fixed wireless networks. The simplest and least expensive diversity method is the position diversity method described above with references to FIG. 1, which nevertheless is not the best possible method for WLL systems.